Related Clues:

• Telephone sound
• It may follow a click
• Open line indicator
• Phone drone
• Phone sound
• Hum on the line
• Caller's go-ahead
• Calling sound
• Cell phone's lack
• 'Go ahead and call' sound
• One thing land-lines have that cellphones lack
• Landline signal
• Sound after a hang-up
• "Ready" signal of a kind
• Hum on a line